1. Field of the Invention
The present invention relates to a semiconductor heterostructure laser cavity more specifically usable in microlasers. These microlasers can be optically or electronically pumped and can emit in a wide wavelength range from the visible to the infrared. The emission range is a function of the materials used for the heterostructure.
More specifically, the laser cavity is pumped by a source outside the cavity more particularly permitting the emission of a visible laser light of 0.4 to 0.6 .mu.m and which has numerous applications. Thus, such a laser can be used for the optical reading and recording of information, e.g. on audio and video compact disks, CD-ROM (compact disks--read-only memories), WORM memories (write once-read memories), erasable memories of the magnetooptical or phase change type), in laser printers and in reprography in general.
It can also be used in other applications such as e.g. in bar code readers, laboratory instrumentation, spectroscopy, biomedical instrumentation, pointers, spectacles, projection display, submarine communications, etc.
For the optical reading and recording of information, the laser cavity according to the invention makes it possible to increase the recording density and simplify the optical instrumentation. In laser printers, the cavity according to the invention permits a better definition of the image and an increase in the printing speed compared with known systems and a better adaptation of the wavelengths to photosensitive materials.
2. State of the Art
The different known semiconductor laser types are injection laser diodes which are the only semiconductor lasers at present on the market, lasers having a laser cavity pumped by an external optical source and lasers, whose cavity is externally electronically pumped. Lasers pumped by an external source have advantages compared with laser diodes, particularly as a result of the separation of the functions and the pumping elements and the laser cavity.
Thus, in injection laser diodes, these basic functions (pumping, cavity) are obtained on the semiconductor by appropriate P and N-type electrical doping of the different epitaxied layers and by ohmic electrical contacting.
The different operations involved in the manufacture of these diodes require a perfect control of the heterostructure production technology and are at present only possible with certain semiconductors from the group of III-V compounds (of type GaAlAs). This limits the wavelength range accessible to these laser diodes to between 0.6 and 1.5 .mu.m.
In external pumping lasers, the injection of the carriers (electrons and holes), which recombine in the active zone of the semiconductor for producing light emission, by definition takes place by a source outside the active semiconductor medium. Consequently it is not necessary to carry out a P or N-type doping of the various epitaxied layers of the laser structure. It is also not necessary for there to be electrical contacts on these layers.
This greatly simplifies the metallurgy of the semiconductor active medium, where consideration only has to be given to the electrical confinement characteristics (electron pumping, quantum wells), optical confinement characteristics (emitted light guidance) and wavelength characteristics.
Thus, it is possible to use in external pumping lasers all direct gap semiconductors and in particular II-VI alloys based on Zn, Cd, Mn, Mg, Hg, S, Se, Te, in which the doping and contact technologies are either not or are only poorly controlled. However, it is not at present known how laser diodes can be made from II-VI material which are equivalent to the known III-V laser diodes. These problems increase as the gap of the materials widens and therefore the emission wavelength shortens. These problems are obviated by the design of external pumping lasers.
The possibility of using all direct gap semiconductors for external pumping lasers makes accessible the wavelength range between the blue and the mid-infrared. In particular, lasers emitting in blue-green make it possible to satisfy existing needs for all applications concerning optical recording. This range is not at present covered by injection laser diodes.
Research is at present taking place for obtaining laser diodes emitting in the blue-green, either on the basis of II-VI semiconductors with the difficulties referred to hereinbefore, or on the basis of III-V laser diodes emitting in the infrared by frequency doubling or similar non-linear effects.
Independently of the pumping mode used, three types of structure are presently used as the active semiconductor medium. These structures can comprise a solid semiconductor material, thin film-type semiconductor materials, or heterostructure-type materials.
The performance characteristics of heterostructure lasers are considerably superior to those of thin film or solid material semiconductor lasers. Heterostructures are widely used in III-V material laser diodes, particularly in the form of a GRINSCH-type structure (graded-index separate-confinement heterostructure) having a graded index optical guide and a separate confinement of the carriers (holes and electrons) and the light.
A GRINSCH-type laser diode structure is described by W. T. Tsang in Appl. Phys. Lett., 39(2), July 1981, "A graded-index waveguide separate-confinement laser with very low threshold and a narrow Gaussian beam", pp 134-136. This known laser diode structure has an active zone constituted by a quantum well located in the centre of a symmetrical composition gradient structure. This quantum well is a thin layer of a semiconductor material with a forbidden band or energy gap below that of the adjacent materials. This composition gradient induces a gap gradient and optical index. The gap gradient improves the collection efficiency of the carriers supplied by the injection current. The index gradient makes it possible to centre the guided optical mode on the active zone. This leads to a good "electron confinement", a good "optical confinement" and an optimum superimposing of the gain zone (quantum well) and the guided optical mode.
The GRINSCH structure makes it possible to obtain a very small laser threshold and it is the "conventional" structure presently used in III-V laser diodes.
Unfortunately, this conventional structure cannot be pumped by an external source because the active zone is much too far from the surface of the structure. The distance separating the active zone from the surface is &gt;1 .mu.m and typically 2 to 3 .mu.m. In addition, the use of N and P doping only makes it possible with considerable difficulty to produce GRINSCH-type laser diode structures from II-VI materials.
A compact heterostructure laser of the GRINSCH type and with external electronic pumping is in particular described in FR-A-2 661 566 filed in the name of the present Applicant. This laser has as the external pumping source an electron gun with a microdot electron source. Such a semiconductor microdot laser or SML has all the advantages of external pumping and the use of a heterostructure referred to hereinbefore. However, this SML requires, in the absence of an adapted, optimized heterostructure, a high operating current in order to reach the laser threshold current density, as well as a high accelerating voltage pumping. Its energy costs can be relatively high, so that it is difficult to produce a compact system with a long life.